What makes you think your pushing the limits of you stock block? You know, you can go pretty damned fast with a well built stocker.
But to answer your question, the biggest give away (at a glance) of a Stage motor in an engine compartment would be the 14 bolt heads. Some Stage motors (Stage-1's) don't have 14 bolt heads, but they have the extra meat in those areas to drill and tap for them. So most people do. If you look down between the header tubes, you will see the additional bolts/studs and nuts.
If you look under the block, you will see the turbo "6" cast into it. (I think this is all of them)
Many stage motors have oil pressure fed from external hoses direct to the block. Some feed to the front. Mine feeds to the back.
Internally, there are a lot of differences. But these things not only just vary from stock blocks, they also vary a lot from other Stage blocks.
All stage blocks have provisions for 4 bolt center mains caps. Some have provisions for all 4 mains to be 4 bolt. Some can be made to have 6 bolts.
There is more beef in a lot of areas.
Some are on center, some are off center.
The list goes on...............
Check this out below, tons of photos and part numbers.
http://www.gnttype.org/techarea/pictureguides/blocks/blockguide.html